In the German patent DE4440613C1 (Spirig, “Vorrichtung und Verfahren zur Detektion eines intensitätsmodulierten Strahlungsfeldes”, 1996), a demodulation device is presented which samples the impinging optical sinusoidally-modulated light signal n times. Charge coupled devices are used for the detection of the light signal and the subsequent transport of the photo-generated charges. Speed limitations are mainly given by the slow diffusion processes during the charge-carrier transport. That problem becomes even more important when the pixel is designed with a large photo-detection region for increased sensitivity.
The German patent application DE19821974A1 (Schwarte, “Vorrichtung und Verfahren zur Erfassung von Phase und Amplitude elektromagnetischer Wellen”, 1999) discloses a photon-mixing element. In order to get a pixel with high-sensitivity and high-speed demodulation facility, a combined structure of stripe-like elements, each of them with short transport paths, is proposed. Nevertheless, the stripe-like structure still leads to a bad fill-factor because the regions between the stripes are not photo-sensitive.
Another approach for large-area demodulation pixel with high sensitivity and high demodulation speed is given in the English patent application GB2389960A (Seitz, “Four-tap demodulation pixel”, 2003). A high-resistive photo-gate of rectangular shape and large size generates a drift-field within the semiconductor substrate enforcing the photo-generated charges to drift to the particular sampling node. Here, any delay of the sampling signal arising on the photo-gate due to large RC-times could reduce the performance of such demodulation pixels. In particular, high frequencies are difficult to realize when many pixels are controlled at the same time. Then the external electronics and their limited driving capability of large capacities represent the constraining factor.
All pixel structures mentioned above have in common that the lateral conduction of the photo-generated charges into a specific direction is always related to the push-pull signal on a gate structure. In order to get higher sensitivities, the photo-detection region has to be enlarged. The results are either increased parasitic capacitances that have to be switched or longer transport paths. Both aspects are undesirable because they restrict the devices to be used with high-frequencies. If the switching gate capacities increase, the speed limitations are given by the driving electronic components. On the other hand, long transport paths reduce the speed of the photo-generated charges in the device making high demodulation frequencies impossible.
The new demodulation device overcomes the problem of making a trade-off between the sensitivity and the demodulation speed. Both aspects can be fulfilled by the pixel at the same time without putting up with any quality loss of the sampling signal or the fill factor. In contrast, the pixel brings up even more advantages that are explained below.